A household electric kettle usually has a heating element which is mounted at the bottom of a water tank through a heat conducting base. A conventional electric kettle typically uses a bi-metal temperature sensor (also known as a thermostat) to sense the water temperature and the steam when the water is boiled, and then cut off the electric power if the water temperature reaches the bi-metal action temperature point or if sufficient amount of steam is detected. Some modern electric kettles use more precise electronic temperature sensors such as the NTC (Negative Temperature Coefficient) resistors (thermistors), which are capable of sensing the water temperature through a very wide range of points instead of just one point. In some cases, the control systems of the electric kettles are configured to take into account the temperature rising rate in addition to the water's absolute temperature value so that the control precision is greatly improved. However, the heating system of an electric kettle is a very complicated energy balancing system. The heat generated by the heating element may be decomposed into the following main components:
a) Energy for heating up the water in the water tank;
b) Energy stored in the heat conducting base;
c) Energy dissipated through the outer case of the water tank and related to the case material and the case mass; and
d) Energy dissipated by emission through the surface the outer case and related to the surface area and the color of the water tank's body.
All of the above factors will affect the water temperature during the heating up and warmth keeping cycle, which may increase the difficulty of precisely controlling the water temperature. For example, the energy stored in the heat conducting base may increase the water temperature by a certain amount after the power supply is cut off, which causes the water temperature to overshoot. The choice of the material of the outer case may affect the water temperature significantly because energy loss is very serious when the water temperature is high. All these factors and effects should be taken into consideration in the design of electric kettles.